It is generally known that antenna performance is dependent on the size, shape, and the material composition of constituent antenna elements, as well as the relationship between the wavelength of the received/transmitted signal and certain antenna physical parameters (that is, length for a linear antenna and diameter for a loop antenna). These relationships and physical parameters determine several antenna performance characteristics, including: input impedance, gain, directivity, signal polarization and radiation pattern. Generally, for an operable antenna, a minimum physical antenna dimension (or the electrically effective minimum dimension) must be on the order of a quarter wavelength (or a multiple thereof) of the operating frequency to limit the energy dissipated in resistive losses and maximize the energy transmitted. Quarter and half wavelength antennas are the most commonly used.
The burgeoning growth of wireless communications devices and systems has created a need for physically smaller, less obtrusive and more efficient antennas that are capable of wide bandwidth operation, multiple frequency band operation and/or operation in multiple modes (e.g., selectable signal polarizations and selectable radiation patterns). The smaller packaging envelopes of current handheld communications devices do not provide sufficient space for the conventional quarter and half wavelength antennas. Thus physically smaller antennas operating in the frequency bands of interest and providing the other desirable antenna operating properties (input impedance, radiation pattern, signal polarizations, etc.) are especially sought after.
Also as is known to those skilled in the art, there is a direct relationship between antenna gain and antenna physical size. Increased gain requires a physically larger antenna, while users continue to demand physically smaller antennas.
U.S. Pat. No. 3,967,276 describes an antenna structure (the so called “Goubau” antenna) comprising four elongated conductors 1, 2, 3 and 4 (see FIG. 1) having dimensions and spacing that are small compared to a wavelength at the applied signal frequency. The conductors are oriented perpendicular to a ground plane 13 with an upper end of each conductor terminated in a conductive plate, identified in FIG. 1 by reference characters 5, 6, 7 and 8. The plates 6, 7 and 8 are oriented parallel to and electrically connected to the ground plane 13 via the conductors 2, 3 and 4. The plate 5 is connected to a signal source (in the transmitting mode) via a conductor 1. In the receiving mode a received signal is supplied to receiving circuitry (not shown), operative with the antenna, via the conductor 1. The plates 5, 6, 7 and 8 are interconnected by inductive elements 9, 10, 11 and 12. The plates 1, 2, 3 and 4 and the inductive elements 9, 10, 11 and 12 can be dimensioned and spaced such that the effective electrical length of the antenna is four times the physical height. For example, if the physical height is 2.67 inches and the wavelength is 60 cm (a frequency of 500 MHz), the effective electrical length is 10.7 cm and the radiation resistance is 50 ohms. Thus the antenna will be balanced to the conventional 50 ohm coaxial cable transmission line. Generally, the plates of such antennas are constructed from sheet metal material, with the elongated conductors comprising conductive wire. These embodiments are relatively expensive to fabricate and clearly are not suitable for use with handheld communications devices.